A type of such diagnostic systems is for example disclosed in U.S. Pat. No. 5,677,611 corresponding to Japanese Patent Application Publication No. H09-23501. A diagnostic system of this type is designed to diagnose that there is an abnormality in a current sensor if the output of the current sensor is zero during a preset period of time. A diagnostic system of another type is disclosed in Japanese Patent Application Publication No. 2005-160136.